Moisture indicator for photoflash lamp

ABSTRACT

A photoflash lamp having a moisture indicator comprising a spot of dried paste disposed on the inside surface of the hermetically sealed lamp envelope to indicate by change in coloration the presence of air contamination. The paste comprises a mixture of cobaltous cobalticyanide and a binder of partially hydrolyzed polyvinyl alcohol.

United States Patent Shaffer Dec. 2, 1975 [5 MOISTURE INDICATOR FOR 3.288,7l8 ll/l966 Carumpalos 252/408 PHOTOFLASH LAMP 3,586,471 6/1971 McDonough ct 252/408 X Primary ExamifierRichard D. Lovering Attorney. Agent. or FirmEdward J. Coleman ABSTRACT A photoflash lamp having a moisture indicator comprising a spot of dried paste disposed on the inside surface of the hermetically sealed lamp envelope to indicate by change in coloration the presence of air contamination. The paste comprises a mixture of cobaltous cobalticyanide and a binder of partially hydrolyzed polyvinyl alcohol.

3 Claims, 2 Drawing Figures US. Patent D60. 2, 197,5

v v a mug MOISTURE INDICATOR FOR PI'IOTOFLASI-I LAMP This is a division of application Ser. No. 211,450 filed Dec. 23, 1971, now US. Pat. No. 3,770,362.

BACKGROUND OF THE INVENTION This invention relates in general to the manufacture of photoflash lamps and similar devices having a sealed envelope, and more particularly to the provision therein of means for determining the presence of air contamination.

Photoflash lamps generate their actinic light output" by the burning of an energetic fuel, such as finely shredded zirconium or aluminum metal, in a combusindicating material to the inner surface of the envelope to indicate, by a color change, whether or not the hermetic closure is intact. The material generally used for this purpose is cobaltous cobalticyanide, which in the anhydrous state is blue and which becomes pink when.

hydrated. The water vapor normally present in air effects a spot color change from blue to pink when the 1 hermetic seal of the lamp envelope is broken.

Cobaltous cobalticyanide is a fine, water-insoluble powder. In order to attach a layer of this material to the inner surface of the glass flashlamp vessel it is ncessary touse it in conjunction with a binding agent. Presently used binding agents include animal glue, colloidal magnesium silicate, and barium chloride. Each of these materials has one or more deficiencies. Natural products such as animal glue vary considerably in properties filled lamps within the range of envelope sizes less than 10 cc. volume. The relatively large envelope volume and manufacturing processes associated with these lamps rendered the thixotropicity problem of little consequence.

More recently, however, the trend in theflashlamp industry has been to the use of subminiature lamps having envelope volumes of less than 1 cubic centimeter.

Such subminiature lamps are presently mass produced in large quantities for use in the small photographic flashlamp units referred to as flashcubes. [n.order to, provide the desired light output levels, these lamps are densely packed with shredded zirconium foil and filled with oxygen at pressures many times higher than atmospheric. In the presence of the significantly increased abrasive action of zirconium, which is further aggravated by the dense packing of the foil, the deficiencies of animal glue and barium chloride are no longer tolerable.

The high degree of thixotropicity developedin the spotting fluids containing colloidal magnesium silicate poses an especially severe problemwith respect to the manufacture of subminiature lamps. Firstly, it is very difficult to even apply the resultant thick paste on the inner surface of the subminiature envelope. The usual method of spot application is by use of a wire dip stick which is initially dipped into the spotting fluid and then insertedinto the. glass envelope to deposit a spot of the indicatingpaste on the,insi de surface thereof. With a spottingfluid containing a binder of colloidal magne- A sium silicate, however, the pasteon the dip stick becomes so thick due to rapid evaporation that it befrom one lot to another. Animal glue sometimes gels,

is subject ,to bacterial spoilage, and does not strongly adhere to glass. Shredded zirconium metal is abrasive like steel wool. Spots aresometimes abraded off of the glasswall during insertion of such shreds into the lamp. Saltssuch as barium or strontium chloride give soft,

easily abraded spots. The low viscosity of spotting fluids using salts as binding agents permits rapid settling of the cobaltous cobalticyanide so that continuous stirring must be used. Colloidal magnesium silicate bonds quite strongly to glass and gives abrasion resistant spots. However, the high degree of thixotropicity developed in such spotting fluids makes application of a thin layer nearly impossible in small diameter lamp vessels,

with the undesirable result of a thick spot deposit which is very slow drying.

Despite these recognized shortcomings, the use of some of these binding agents with cobaltous cobalticyanide was quite extensive over the years with many of i to be a useful binding agent in the larger zirconiumcomes difficult, if not possible to insert the..pasteloaded dip stick into the very small opening of the subminiature glass envelope without smearing. Alternative methods of spot application were attempted without success. Secondly, when after considerable difficulty spots of paste are deposited on the envelopes, the spots are intolerably slow drying due to their thickness and are incompatible with the drying times and'processes available on the high speed automated manufacturing equipment employed in producing subminiature lamps.

Further, the air exchange is so slow in such small lamp envelopes that the drying problems are exaggerated.

Accordingly, spotting fluids based on colloidal magnesium silicate are not used in subminiature all-glass envelopes.

."SUMMARY OF THE INVENTION view of the forego'ing, it is an object of this invention. to pr ovide asuperior moisture indicator for photo- A particular 'object of the invention is to provide an improved binding'agent for a photoflash lamp moisture indicator which adheres more strongly to glass and which promotes much greater spot integrity and abrasion resistance, thereby resulting in a satisfactory and reliable air leak indicator for super-atmospheric flashlamps employing zirconium as the combustible fill.

A principal object is to provide an improved moisture indicating material for photoflash lamps which is com.- patible with the high speed automated manufacture of subminiature flashlamps.

These and other objects, advantages and features are attained, in accordance with the principles of this invention, by using partially hydrolyzed grades of polyvinyl alcohol as the binding agent for moisture indicating spots in flashlamps. These resins have been found to provide excellent spot adhesion and abrasion resistance and are available in a series of viscosity ranges that permit spotting fluids of any desired viscosity and solids content to be formulated. Polyvinyl alcohol solutions have excellent pigment suspension characteristics and, being nonthixotropic, they permit a controlled spot layer thickness to be applied. Realistic drying rates are thereby attainable, and the spotting fluid is rendered suitable for the dip stick method of application to the inside of subminiature lamp envelopes. The degree of adhesion and abrasion resistance attained by the use of partially hydrolyzed polyvinyl alcohol was unmatched by any other material that I tested.

BRIEF DESCRIPTION OF THE DRAWING This invention will be more fully described hereinafter in conjunction with the accompanying drawings, in which:

FIG. 1 is an elevational view of an electrically ignitable photoflash lamp provided with a moisture indicator in accordance with the principles of the invention; and

FIG. 2 is a sectional elevation of a percussive-type photoflash lamp provided with a moisture indicator, according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENT The teachings of the present invention are applicable to either percussive o'r electrically ignited photoflash lamps of a wide variety of sizes and shapes, however, the invention is particularly advantageous as applied to flashlamps having envelopes with a volume of less than one cubic centimeter (cc.). Accordingly, FIGS. 1 and 2 respectively illustrate electrically ignited and percussive type photoflash lamp embodying the principles of the invention.

Referring to FIG. 1, the electrically ignitable photoflash lamp illustrated therein comprises an hermetically sealed light-transmitting lamp envelope 2 of glass tubing having a press 4 defining one end thereof and an exhaust tip 6 defining the other end thereof. Supported by the press 4 is an ignition means comprising a pair of lead-in wires 8 and 10 extending through and sealed into the press. A filament 12 spans the inner ends of the lead-in wires, and beads of primer 14 and 16 are located on the inner ends of the lead-in wires 8 and 10 respectively at their junction with the filament. Typically, the lamp envelope 2 has an internal diameter of less than one-half inch, and an internal volume of less than 1 cc. A quantity of filamentary combustible material 18 such as shredded zirconium foil, is disposed within the lamp envelope. The envelope 2 is also provided with a filling of combustionsupporting gas, such as oxygen for example, at a pressure of several atmospheres. Preferably, the combustible and the combustion-supporting gas are substantially in stoichiometric balance.

The percussive-photoflash lamp illustrated in FIG. 2 comprises a length of glass tubing defining an hermetically sealed lamp envelope 22 constricted at one end to define an exhaust tip 24 and shaped to define a seal 26 about a primer 28 at the other end thereof. The primer 28 comprises a metal tube 30, a wire anvil 32 and charge of fulminating material 34. A combustible such as filamentary zirconium 36 and a combustionsupporting gas such as oxygen are disposed within the lamp envelope, as described for the electrically ignited lamp. The wire anvil 32 is centered within the tube 30 and is held in place by a circumferential identure 38 of the tube 30 which laps over the head 40 or other suitable protuberance at the lower extremity of the wire anvil. Additional means, such as lobes 42 on wire anvil 32 for example, may also be used in stabilizing the wire anvil, supporting it substantially coaxial within the primer tube 30 and insuring clearance between the fulminating material 34 and the inside wall of the tube 30. A refractory bead 44 is fused to the wire anvil 32 just above the inner mouth of the primer tube 30 to eliminate burn-through and function as a deflector to deflect and control the ejection of hot particles of fulminating material from the primer. The lamp of FIG. 2 is also typically a subminiature type having envelope dimensions similar to those described with respect to FIG. 1.

Although the lamp of FIG. 1 is electrically ignited, usually from a battery source, and the lamp of FIG. 2 is percussion-ignitable, the lamps are similar in that in each the ignition means is attached to one end of the lamp envelope and disposed in operative relationship with respect to the filamentary combustible material. More specifically, the igniter filament 12 of the flash lamp in FIG. 1 is incandesced electrically by current passing through the metal filament support leads 8 and 10, whereupon the incandesced filament ignites the beads of primer l4 and 16 which in turn ignite the combustible 18 disposed within the lamp envelope. Operation of the percussive-type lamp of FIG. 2 is initiated by an impact onto tube 30 to cause deflagration of the fulminating material 34 up through the tube to ignite the combustible 36 disposed within the lamp envelope.

In the manufacture of photoflash lamps of the type just described, the lamp envelope 2 or 22 is initially a segment of glass tubing, open at both ends. In the normal sequence of operations, the mount structure which comprises filament l2 and lead-in wires 8 and 10, or the primer 28, is positioned in one of the open ends of the glass tubing and the press 4 or seal 26 is formed, thereby closing one of the open ends of the tubing. A suitable applicator is then inserted into the remaining open end of the tubing to provide the inner wall thereof with a moisture indicator spot, denoted as 20 in FIG. 1 and 21 in FIG. 2. Thereafter, the paste spot 20, or 21, is dried, and a charge of combustible, in this instance shredded zirconium foil, is introduced into the remaining open end of the tubing. The glass is then constricted at the open end, and the envelope is exhausted, filled with oxygen gas at several atmospheres pressure, and tipped off at 6 to define a hermetically sealed envelope. Thereafter, the spot of dried paste 20, or 21, which normally has a blue appearance, functions to indicate, by a change in coloration to pink, the presence of an air leak in the envelope.

The photoflash lamp illustrated in FIG. 2 is of the type identified commercially as an MC-l and which is used in percussive-flashcubes of the type described in US. Pat. No. 3,597,604. The lamp envelope volume of a MC-l is about 0.68 cc.; the gas pressure is about 550 cm. of Hg; and the quantity of shredded zirconium foil with which the lamp is provided is about 29 mgs. per cc. of envelope volume.

In accordance with the present invention, the dried paste of the moisture indicator spot 21 comprises a mixture of cobaltous cobalticyanide and a binder of partially hydrolyzed polyvinyl alcohol. Upon use in thousands of lamps of the MC-l type described, this composition has exhibited excellent spot adhesion and abrasion resistance characteristics. Further, as the partially hydrolyzed grades of polyvinyl alcohol are commercially available in a series of viscosity ranges, wide flexibility is permitted in the choice of solids content and viscosity of the spotting fluid formulation prepared by stirring the aforementioned mixture in water. The polyvinyl alcohol solutions exhibit excellent pigment suspension characteristics and are non-thixotropic. Accordingly, the use of partially hydrolyzed polyvinyl alcohol as the binding agent for the moisture indicator is readily adapted to the dip stick method of spot application, permits a controlled spot layer thickness to be applied, and thereby provides realistic drying rates.

Polyvinyl alcohol is prepared by hydrolytic removal of acetyl groups from polyvinyl acetate. Two types of polyvinyl alcohol are commercially available-completely and partially hydrolyzed grades. The completely hydrolyzed grades have 97% or more of the acetyl groups removed, whereas the partially hydrolyzed grades retain from to of the acetyl groups.

The completely hydrolyzed grades of polyvinyl alcohol provide good spot adhesion and abrasion resistance, but the moisture sensitivity of the completely dried spot is poorer. For that reason, previous investigations in search of a better binding agent for use in moisture indicators for photoflash lamps rejected polyvinyl alcohol as unsuitable for the application. Consequently, it was with considerable surprise that l discovered the superb suitability of the partially hydrolyzed grades of polyvinyl alcohol as a binding agent for the moisture indicating material; these particular grades of polyvinyl alcohol not only provide the advantages outlined above, but when used in proper quantities as a binding agent, the moisture indicating substance functions at a sensitivity comparable to that of the best prior art formulations.

Other materials evaluated as possible binding agents for moisture indicating spots included hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, poly(ethyleneoxide), and polyvinyl pyrrolidone. None of these materials approach the degree of adhesion and abrasion resistance attained by polyvinyl alcohol. Some materials were not sufficiently permeable to moisture to give good spot sensitivity.

By way of example, 1 have found that a highly satisfactory moisture indicator may be prepared by combining a mixture of about 10% of partially hydrolyzed polyvinyl alcohol and cobaltous cobalticyanide by weight in an appropriate vehicle such as water. Sufficient water is stirred into the mixture to give the desired percent of solids. Viscosity is controlled by the grade of partially hydrolyzed polyvinyl alcohol resin chosen as well as the relative quantity of water used. A resin that gives a viscosity of about 4 to 6 centipoise when diluted to about 4% by weight in water is atisfactory.

The partially hydrolyzed polyvinyl alcohol may comprise from about 1 to 20% of the dried spot by weight. At very low values, adhesion and spot integrity decreases, whereas at high binder contents the spot sensitivity falls off rapidly.

What I claim is:

l. A moisture indicating material for disposition upon a glass surface within the sealed envelope of a lamp to indicate the presence of air therein, said indicating material consisting essentially of a mixture of cobaltous cobalticyanide and between about 1% to about 20% by weight of partially hydrolyzed polyvinyl alcohol.

2. The moisture indicating material of claim 1 wherein said mixture comprises about 90% of cobaltous cobalticyanide and about 10% of partially hydrolyzed polyvinyl alcohol by weight.

3. The moisture indicating material of claim 1 wherein said partially hydrolyzed polyvinyl alcohol is of a type which yields a viscosity of about 4 to 6 centipoise when diluted to about 4% by weight in water. 

1. A MOISTURE INDICATING MATERIAL FOR DISPOSITION UPON A GLASS SURFACE WITHIN THE SEALED ENVELOPE OF A LAMP TO INDICATE THE PRESENCE OF AIR THEREIN, SAID INDICATING MATERIAL CONSISTING ESSENTIALLY OF A MIXTURE OF COBALTOUS COBALTICYANIDE AND BETWEEN ABOUT 1% TO ABOUT 20% BY WEIGHT OF PARTIALLY HYDROLYZED POLYVINYL ALCOHOL.
 2. The moisture indicating material of claim 1 wherein said mixture comprises about 90% of cobaltous cobalticyanide and about 10% of partially hydrolyzed polyvinyl alcohol by weight.
 3. The moisture indicating material of claim 1 wherein said partially hydrolyzed polyvinyl alcohol is of a type which yields a viscosity of about 4 to 6 centipoise when diluted to about 4% by weight in water. 